In Medical Instrumentation applications using ultrasonic energy as means to perform work, many solutions have been presented for different constitutive building blocks like current/voltage feedback loops, driver Amplifier (single stage, push-pull, half bridge, full bridge). In bone fracture healing and various surgical Instruments a wide load variation is expected and for safety and reliability reasons a fast response to fast load change is desired. These Instruments use piezoelectric single elements or stacks as load for the power amplifier. For low acoustic power application, in general less than 5 Watts a single element is used, for higher power, for example higher than 20 Watts a stack of elements is preferred. The electrical parameters [8], [9] are well defined by the manufacturer and they depend on the mechanical action that they have to perform. The mechanical work varies widely and due to the interdependence between electrical and mechanical parameters, the Power Amplifier driving the piezoelectric load “sees” a continuously varying electrical load. Fast reaction to fast load change is therefore something desirable. Typical piezoelectric constructions in Medical applications can be exemplified in U.S. Pat. No. 4,530,138 or U.S. Pat. No. 3,889,166
1. Field of the Invention
The invention in general relates to Medical Instruments delivering ultrasonic energy to perform work and more particularly to improvements related to load monitoring, by implementing a method to digitally measure the distance from optimal load conditions and also a method to fast react (and shut down if necessary) due to load changes in one clock cycle of the Power Amplifier if the load changes in one cycle.
2. Description of Prior Art
For cutting and cauterization applications, In Surgical Instrument presented in U.S. Pat. No. 7,273,483 a solution is offered for an environment where transducer temperature increase can lead to material fatigue and possible failure. Mainly for adjusting Power to the load via a push-pull amplifier, it measures load conditions via current averaging and voltage averaging followed by analog to digital (ADC) conversion in a feedback loop control scheme. Reaction time to load change is thus determined by current and voltage averaging filters plus ADC conversion time, too slow for a sudden load change which could lead to Instrument failure. A similar feedback approach is adopted in U.S. Pat. Nos. 5,026,387 and 4,056,761. These Instruments cannot react fast enough to prevent a catastrophic load condition which could lead to shatter or breakage.
In Exogen 2000+SAFHS, a Pulsed Ultrasound Instrument for bone fracture healing manufactured by Smith and Nephew located in Memphis, Tenn., a generic averaging filter is used to determine transducer electrical impedance variation due to the amount of coupling gel used and thus determining gel/no gel alarm conditions (U.S. Pat. No. 6,261,249). These Pulsed Ultrasound Instruments under sudden load change can also lead to Instrument failure. Therefore there is a need for a fast Instrument response due to fast load change. In this application a Class E Power Amplifier driving a piezoelectric load is used as Power Stage. Class E amplifier is well known. Fast feedback Class E operation is also not new. A generic Class E amplifier as final power stage and a predictor to load change was presented in [1]. It was tested by slightly adjusting the frequency of operation. In reference [1] a resistor is placed in series with the source of power transistor Q1 and an additional circuit is built to indicate class E or non class E operation, however such a circuit has a time response of 20 to 30 ms when operating at 1 MHz which in certain situations like highly load sensitive ultrasonic surgery instruments and Pulsed Ultrasound can be too slow. FIG. 4A in U.S. Pat. No. 3,919,656 and reference [1] FIG. 2 present typical Class E waveforms under variable load quality factor (Q). Design of Class E power Amplifiers under nominal or off nominal (variable duty cycle D, or duty ratio as it is considered in [2], [3]) is also known.